How can I modify a AC light dimmer to do a continuous sweep between bright and dark. I want to be able to vary the speed and possibly have adjustable range limits for the bright peak and the dark minimum. <p>I'm at a loss as to how to do this so any advice is appreciated. Or even an example circuit.<p>Thanks,
Todd Snyder

The simple way is to use a VCR or Tape Recorder Tiny motor and pulley assembly, attached to a bell crank. Motor speed can be with a simple pot, and a bell crank can be carved out of a large plastic pulley, commonly found in VCRs etc.<p> Perhaps you can find a nice reduction ratio pulley platter inside the VCR that will reduce the rotation to a very slow speed, similar to the Eject gear mechanism of the common cassette and VCR.<p> Or Take it out whole and use the eject mechanism which is usually gear driven, but with out limit switches it goes in and returns out like a piston as long as you supply power to it.

Thanks for your reply...funny but the "mechanical" solution you mention is the first one that seems to come to everyones mind (from another forum I've asked this question on as well). <p>I would really like to think of an electronic solution. <p>The way I see the problem is that we need to find a way to effectively (electronically) change the pot resistance. How about replacing the pot with two power transitors each emmiter attached to the others collector. Then bias the transistor into their linear ranges and apply a sine wave at the frequency (and magnitude) that would effectively make the transitors look like variable resistors?<p>What are the "gotcha's" with this approach? Would we need transistors that can handle a reverse and forward voltages of 200V?

That would be some big bucks there wouldnt' it be? ....I would motorize some cheap dimmer switches I think before dedicating a variac for this purpose.<p>I just want some Ideas on how to implement this with a triac. I guess it must be pretty problematic as no one has any "electronic" ideas for this. <p>I did a search for info on here at N&V and another guy asked the same question a while back and got about the same responses. "motorize your dimmer swtich".<p>Thanks for the variac idea. <p>Does anyone think using opposing transistors would work (as I tried to describe above)?

The pot in a light dimmer just sets the voltage going to the gate of an SCR. When the voltage is changed so does the point in the AC wave form at which the SCR goes conductive and applies power to the lamp.<p>You just want to replace the pot with a voltage signal.<p>Pay careful attention to all the potentials involved. I believe a proper circuit would include opto or other kinds of isolation.

I guess I just need to do a little probing in there to see what voltages we're talking about.<p>Will this dimmer work in the neutral leg of the lamp so that I can use my O scope closer to ground levels...do you know?

Maybe you could use an ldr with an led afixed to it and then cover them up so no external light can get in. Then when you vary the intensity of the led the resistance of the ldr would change giving you control over the dimer.(I am speaking of the slide type dimmers) You would replace the slide with the ldr,led assembly.<p>I know this will work because I have one, no really I do, my wife made it after I tried for a week to get the motorized one to work, it was time to mow the yard and I kept saying just as soon as I get this to work, so, in about 10 minuets she made one that worked. wait should I have told that story? come to think of it I made the one that worked and she mowed the yard! <p> web page

Some 20 yrs.ago, there was a touch dimmer which fit in a wall-switch box, and had a flat,oval, brass touch plate. When you touched the plate, the light would cycle (as you've spec'd), until you released your finger (at the desired brightness). WELL, as kids, we found that holding your finger kept it cycling, and in hot weather it'd continue a second. A lit match kept it cycling about 2-minutes!! Check older hardware stores, in their "dusty-old-inventory" area.

Thanks, yes I had seen a hack for doing just that but couldn't remember what it was. At first I thought it was a hack on a normal dimmer but after puzzleing on it a while I remembered the hack was for a touch dimmer. The hack was to add a 47pf cap from the touch plate to ground. <p>Those touch dimmers have a chip inside that does the logic. I dont know if modern touch dimmers will cycle back and fourth but the Hack page called ou a levithon model (about $20). I may see what I can find next time I'm at Lowes.<p>Thanks for your input.<p>Todd Snyder